Solar cells made of carbon only are becoming a possibility

(Nanowerk Spotlight) A few days ago we reported on how graphene is being explored, among many other uses, as a replacement material for silicon in photonic devices to expand the wavelength range of operation and to improve performance ("Graphene photodetectors for high-speed optical communications"). Another area where graphene potentially could replace silicon and could have a huge impact is as light-absorbing material in solar cells. Although it has been well known that graphene has very attractive properties for photovoltaic applications – tunable bandgap and large optical absorptivity – these advantages could not be exploited so far due to the problem of fabricating solution-processable, stable, and large enough, size-controllable graphene structures useful for charge collection in solar cells.

Graphene quantum dots (see: "Graphene quantum dots as single-electron transistors") might offer a solution to these fabrication problems. Researchers have now developed a fabrication technique that, combined with well-developed carbon chemistry, enabled them to synthesize solution-processable, black graphene quantum dots with uniform size through solution chemistry. They have also demonstrated that these graphene quantum dots can be used as sensitizers for solar cells.

"Chemists and engineers experimenting with graphene have come up with a whole host of strategies for keeping single graphene sheets separate," Liang-shi Li, an Assistant Professor of Chemistry at Indiana University, tells Nanowerk. "Prior to our work, the most effective solution has been breaking up graphite (top-down) into sheets and wrap polymers around them to make them isolated from one another. But this makes graphene sheets with random sizes that are too large for light absorption for solar cells. We tried a different idea: By attaching a semi-rigid, semi-flexible, three-dimensional sidegroup to the sides of the graphene, we were able to keep graphene sheets as big as 168 carbon atoms from adhering to one another."

"To our knowledge, these are the biggest stable graphene sheet ever made with the bottom-up approach" says Li.

To reduce the tendency of forming insoluble aggregates by large graphene nanostructures, Li and his team have developed a strategy to shield the graphenes from one another by enclosing them in all three dimensions.

Strategy to make large graphene quantum dots soluble. (a) Attaching 1,3,5-trialkyl phenyl moieties (marked black) covalently to the edge of graphene (blue) to shield the graphenes from one another in three dimensions. Hydrogen atoms are marked white. (b) Molecular structure of graphene quantum dot 1, containing a graphene moiety with 168 conjugated carbon atoms. The graphene moiety is marked blue and the three solubilizing groups black. (c) A theoretically energy-minimized configuration of 1 in vacuum, showing the three-dimensional enclosure of the graphene core by the alkyl chains (black). (Reprinted with permission from American Chemical Society)

"We achieve this by covalently attaching multiple 1,3,5-trialkyl-substituted phenyl moieties (at the 2-position) to the edges of the graphenes," explains Li. "The crowdedness on the edges forces the peripheral phenyl groups to twist from the plane of the graphene, resulting in the alkyl chains at 1,3-positions extending out of the plane and the one at 5-position extending laterally. This leads to increased distance between graphenes in all three dimensions and thus greatly reduces the intergraphene attraction due to its short-range. This approach is reminiscent of intercalation of atoms or molecules in graphite that significantly reduces the interlayer binding energy."

The sidegroup consists of a hexagonal carbon ring and three long, barbed tails made of carbon and hydrogen. Because the graphene sheet is rigid, the sidegroup ring is forced to rotate about 90 degrees relative to the plane of the graphene. The three brambly tails are free to whip about, but two of them will tend to enclose the graphene sheet to which they are attached.

The tails don't merely act as a cage, however. They also serve as a handle for the organic solvent so that the entire structure can be dissolved. Li and his colleagues were able to dissolve 30 mg of the species per 30 mL of solvent.

To test the effectiveness of their graphene light acceptor, the scientists constructed rudimentary solar cells using titanium dioxide as an electron acceptor. They were able to achieve a 200-microampere-per-square-cm current density and an open-circuit voltage of 0.48 volts. The graphene sheets absorbed a significant amount of light in the visible to near-infrared range (from about 200 to 900 nm) with peak absorption occurring at 591 nm.

"Our fabrication technique opens exciting opportunities to tune the optical and electronic properties of graphenes for photovoltaics" says Li. "Moreover, both theoretical and experimental studies have shown that the geometry and chemical nature of the edges in graphene nanostructures play an essential role in determining their electronic and magnetic properties. The solution-chemistry approach to stable graphenes with structural control on a molecular level should enable us to test these theoretical predictions."